﻿/* 4. Write a program that calculates N!/K! for given N and K (1<K<N). */

// Add Reference to 'System.Numerics'.

using System;
using System.Numerics;

public class FirstExpressionInvolvingFactorial
{
    public static int k;
    public static int n;

    public static void Main()
    {
        BigInteger result = 1;

        DataInput();

        for (int i = n; i > k; i--)
            result *= i;

        Console.WriteLine("N!/K! = {0}", result);
    }

    public static void DataInput()
    {
        do
        {
            Console.Clear();
            Console.Write("K = ");
            if ((int.TryParse(Console.ReadLine(), out k)) && (1 < k))
            {
                Console.Clear();
                Console.WriteLine("K = {0}", k);
                Console.Write("N = ");
                if ((int.TryParse(Console.ReadLine(), out n)) && (k < n))
                    return;
            }
        } while (true);
    }
}